A spectrometer is an optical device for decomposing light to be measured into individual spectral components by using a spectroscopic portion such as a prism and a diffraction grating (refer to Patent Document 1, for example). According to the above-described spectrometer, a light detecting element is used to detect spectral components of the light dispersed by the spectroscopic portion, thus making it possible to know the wavelength distribution of light, the intensity of a specific wavelength component and others.    Patent Document 1: Japanese Published Unexamined Patent Application No. H08-145794